The present invention relates to an endoscope objective lens system that is effectively compensates for lateral chromatic aberration.
Objective lenses for use in endoscopes require that their outside diameter and overall length be made as compact as possible in consideration of the fact that they are assembled in the tip portion of an endoscope together with illumination optics and air/water supplying channels.
The assignee of the present invention previously filed Japanese Patent Application No. 117629/1987 on May 14, 1987 and proposed an invention of a compact endoscope objective lens system of a three-group-four-element composition (this invention is hereunder referred to as prior invention A). A diagrammatic cross section of this lens system is shown in FIG. 26. The same assignee improved the prior invention A and filed Japanese Patent Application No. 125381/1987 on May 22, 1987, in which they proposed an invention of a lens system characterized by a further improvement in lateral chromatic aberration (this invention is hereunder referred to as prior invention B). A diagrammatic cross section of this lens system is shown in FIG. 28.
The lens systems proposed by the two prior inventions A and B employ the necessary minimum number of lens elements (three-group-four-element composition) in order to attain compactness, so they have only a cemented positive lens disposed on the image side to effect achromatism and the result is not necessarily satisfactory in terms of compensation for lateral chromatic aberration.
As shown in FIGS. 26 and 28, the prior inventions A and B depend on providing a deep interface in the cemented positive lens with a view to achieving achromatism. In the prior invention A (FIG. 26), the center of curvature of the interface is on the side of a diaphragm stop, so extra-axial rays of light encounter the interface in a near-perpendicular direction and the difference in refractive index that is caused by the difference in wavelength is not substantial. This is effective for the purpose of compensating for longitudinal chromatic aberration but not so effective in compensation for lateral chromatic aberration. If the curvature of the interface is increased in order to ensure more effective compensation for lateral chromatic aberration, the degree of meniscus of the negative lens element in the cemented lens is increased and the thickness of the peripheral portion of the positive lens element is decreased, making the manufacture (or working) of a small endoscope objective lens either difficult or entirely impossible. If a thicker lens is used in order to ensure a peripheral portion thick enough to be worked, the overall size of the optics is increased and its composition is not at all suitable for use in an endoscope.
In the prior invention B (FIG. 28), the center of curvature of the interface is positioned on the side opposite to the side of a diaphragm stop, so extra-axial rays of light encounter the interface at an angle way off the perpendicular direction. As a result, the difference in refractive index that is caused by the difference in wavelength is increased, which is effective in compensating for not only longitudinal chromatic aberration but also lateral chromatic aberration. However, if an attempt is made to further increase the effectiveness of this lens system, problems can occur as in the case of the prior invention A, such as increased difficulty involved in fabricating positive and negative lens elements or the increase in the overall size of the optics. In addition, the angle of incidence of extra-axial rays will increase with respect to the line normal to the interface, thereby increasing the chance of the occurrence of coma. Furthermore, total reflection becomes highly likely to occur and in order to avoid it, some design compromise must be made by adopting a suitably large radius of curvature of the interface.
In order to achieve achromatism, another cemented lens might be incorporated in the lens system of the prior invention A or B. This would not be the best solution, however, since the most important requirement for an endoscope objective lens system is its compactness.
As described above, attempts to achieve further improvements in compensation for lateral chromatic aberration in the prior inventions A and B have caused problems in association with lens fabrication and the size of optics.
With objective lenses for use in endoscopes, it is desired that lateral chromatic aberration is minimized with reference to the distance between optical fiber cores (or a dimension corresponding to one pixel in an imaging device in an electronic endoscope using the imaging device). If lateral chromatic aberration is compensated insufficiently, the image transmitted will produce noticeable color diffusion, especially in the marginal portion, and the resulting deterioration of image quality will not only cause perception of a defective image but also prevent correct viewing (diagnosis in medical applications).
Endoscopes generally employ optical fibers spaced at a core-to-core distance of about 10 .mu.m. But with the recent advances in fiber production technology, fibers of smaller diameter have become available and the resolution of fibers per se as image guides is becoming correspondingly higher. A need therefore has arisen to minimize the lateral chromatic aberration of an endoscope objective lens so as to achieve high image quality (i.e., high contrast and high resolution).